I'm so sorry to hear about your loss. That is awful.
Yes, the lease is still valid even though your brother, one of the tenants, passed away.
Typically, when a party to a contract dies, the estate of the deceased picks up and receives the rights and obligations to the contract in the place of the deceased person.
If the deceased person's estate wants to step in and be jointly responsible for paying the rent (and has sufficient funds or assets to do that), you are probably okay. You most likely need to discuss the situation with your landlord and let him know how you wish to proceed. If the lease is for a period of time - like a year - and that year is not up yet, and as long as the income requirements were not in the contract, then the landlord should not be able to have you evicted.
Of course, you and your brother are both probably jointly liable for the rent, and so if the entire amount is not paid, then the landlord will have a cause of action to evict you, so if you can no longer afford the apartment, you will probably need to start looking for other options, or look for another roommate.
Again, I'm so sorry for your loss. Please let me know if this has been helpful.